There will be times in your iPhone's life when you'll need to reset the device. You have two ways to do that — a soft reset and a factory reset — and knowing when to use either method is nearly as important as knowing how.
Soft-resetting Your iPhone
Soft-resetting your iPhone is simply a way to restart the device. You don't delete any data at all. It can be a handy way to resolve problems with an erratically behaving iPhone. If apps are crashing, your phone can't recognize connected device that it's worked on before or your iPhone completely locks up, a soft reset can set things right.
To soft reset your iPhone, press and hold the Home button and the Sleep/Walke button simultaneously for at least 10 seconds. You can let go of both buttons when you see the Apple logo appear on the iPhone's display. Your iPhone will go through its startup routine and after a while — 30 to 45 seconds, usually — it will go back to your home screen. If you've set up a passcode, you'll need to re-enter it after resetting your iPhone.
Resetting Your iPhone to its Factory Settings
If you've inherited a second-hand iPhone, or if you're getting ready to sell or give yours away so that you can upgrade to a new iPhone 8 or iPhone X, you may want to reset it to make it as "factory fresh" as possible.
Take note that this deletes all information off of your iPhone. If you have any irreplaceable data — photos, contact information or important messages — make sure your iPhone is backed up first or make sure that information is stored safely in iCloud before proceeding. You can use iTunes on Windows or OS X to back up the iPhone to your computer.
If you want to delete everything on your iPhone and start over again, here's how.
1. Open Settings.
2. Tap General.
3. Tap Reset. Reset is located at the bottom of the General settings screen.
4. Tap Erase All Content and Settings. You may get a warning that your documents and data are uploading to iCloud.
5. When you're prompted, enter your passcode.
6. Tap Erase iPhone. You'll need to do it again to confirm you really want to erase the iPhone, and you'll be warned that this action cannot be undone.
7. Enter the password for your Apple ID to remove the iPhone's information from your account. Click Erase to continue the process.
Your iPhone will restart, and will begin to reset. You'll see a blank screen with an Apple logo and a progress bar. This process takes several minutes.
Once your iPhone is reset, all of your media and data will be gone, and your iPhone will set up the same way it was when you first took it out of the box.
Bear in mind that you can also reset your phone's settings without deleting the files stored on your phone, including songs, pictures and movies. You can also just delete network settings, and reset specific settings including the keyboard dictionary, home screen layout, and location & privacy information. To do so, select the appropriate option in the Reset menu.
None of these other options will delete any files from your iPhone — they just simply restore settings to their default positions. Reset Keyboard Dictionary does delete all the custom words you've typed on the keyboard, however, thus restoring the keyboard dictionary to its factory default.